Thin films of high transition temperature (high T.sub.c) superconductors are becoming increasingly important in the study of basic superconducting properties, and have significant potential for application to electronic devices. For example, thin films of c-axis oriented Y-Ba-Cu-O consisting of only a few unit cells in one dimension make it possible to study the influence of a limited number of Cu-O planes on its superconducting properties. See, for instance, X.X.Xi et al., Applied Physics Letters, Vol. 54, No. 23, pp. 2367-2369, 1989. Since thin metal oxide films can be fabricated that have high transition temperatures similar to those of bulk structures, the use of these films in bolometers and other sensors, SQUIDS, RF striplines, superconducting FETs, and interconnect technology is possible. See, for instance, Y. Katoh et al., Japanese Journal of Applied Physics, Vol. 27, No. 6, pp. L1110-L1112, 1988. However, to achieve these applications, it is necessary to develop appropriate microfabrication techniques.
Thin superconducting Y-Ba-Cu-O films have been fabricated by a variety of techniques, including magnetron sputtering (see, for instance, H. Tsuge et al., Japanese Journal of Applied Physics, Vol. 27, No. 11, pp. L2237-L2239, 1988), laser evaporation (see, for instance, D. Dijkkamp et al., Applied Physics Letters, Vol. 51, No. 8, pp. 619-621, 1987) and e-beam evaporation (see, for instance, B. Oh et al., Applied Physics Letters, Vol. 51, No. 11, pp. 852-854,1987). A stoichiometric form of Y-Ba-Cu-O commonly fabricated is YBa.sub.2 Cu.sub.3 O.sub.7.
Superconducting YBa.sub.2 Cu.sub.3 O.sub.7 is a metal oxide which can be visualized as having layers and chains of copper and oxygen. The layered structure has parallel planes of Cu-O sheets. The Cu-O planes are perpendicular to the c-axis, the c-axis being defined as the crystal axis normal to the basal plane of the unit cell. A compound that can be described as comprising parallel planes of its constituent elements layered one plane on top of another plane is said to have a planar structure.
A thin film with a c-axis orientation has its surface perpendicular to the c-axis of the material, i.e., its surface is parallel to the basal plane of the material. Similarly, a thin film with an a-axis (b-axis) orientation has its surface parallel to the c-axis with the a-axis (b-axis) perpendicular to the surface. For optimal charge transport in the plane of a superconducting film, c-axis orientation is the preferred orientation (see, for instance, P. M. Mankiewich et al., Applied Physics Letter, Vol. 51, No. 21, pp. 1753-1755, 1987).
As a layer of c-axis oriented superconducting metal oxide, such as YBa.sub.2 Cu.sub.3 O.sub.7 is deposited on a substrate, the oxide changes with increasing thickness from a substantially 100% c-axis oriented material to a mixture of c-axis and a-axis oriented material. This causes the film surface to become disordered due to the orthogonality of the Cu-O sheets between the two orientations, and to the random distribution of the orientations across the film surface. A disordered surface typically tends to be rough and uneven. In addition, disorder due to a misorientation can result in a surface being structurally of relatively low crystalline quality (see, for instance, X.X.Xi et al., op. cit.).
A rough and uneven surface makes device processing difficult, since device processing typically involves deposition of a layer (layers) of material on the surface. A rough surface also results in a superconductor film having a relatively high RF loss. In addition, a rough junction interface inhibits proper operation of a superconducting tunneling junction. Film deposition at lower processing temperatures helps to reduce, but does not eliminate, the problem.
In view of the need for smooth, thin high T.sub.c superconductor films for device applications, a method for fabricating thin films of metal oxide superconductors of a desired thickness and having a smooth surface would be highly desirable. This application discloses such a method.